The present invention relates to an adjustable video signal gamma controller for a television system.
In a television system it is necessary to process the video signals (e.g., before transmission) to compensate for certain nonlinearities in the respective transmission and receiving systems to ensure that viewers see a picture which is a true reproduction of the televised scene. Among the nonlinearities of the system for which compensation must be made are the gamma characteristics of the pickup tube of the television camera and the picture tubes of television receivers.
Although a television camera including solid state imagers, such as a charge-coupled device (CCD), has a substantially linear optical-to-electrical conversion characteristic, gamma control is still desirable to correct for signal processing nonlinearities in the video processor of the camera as well as nonlinearities of the picture tubes of television receivers. For example, if a video signal varying linearly in amplitude is applied to a conventional picture tube in a television receiver, it is likely to result in a picture having a contrast range which is undesirably distorted in accordance with the nonlinear electrical-to-optical conversion characteristic (gamma) of the picture tube. A measure of the nonlinearity is the exponent of the power law function which describes the nonlinear transfer characteristic. Accordingly, it is desirable to adjust the gamma of the video signal prior to transmission in such a manner that the signal received and reproduced in a television receiver has an undistorted contrast range.
Generally, gamma correction is accomplished by passing the video signals derived from the television camera through a nonlinear circuit having a predetermined power law relationship (i.e., y=x.sup..gamma.) between its input and output to compensate for the substrate nonlinear transfer characteristic of the picture tube in the television receiver. It is generally accepted that the power law relationship provided by the nonlinear circuit should have an exponent (or power) which is the reciprocal of the picture tube gamma (e.g., +2.2). Typically the exponent is 0.45. The gamma correction circuit is usually located in a video signal processing amplifier coupled between the camera pickup tube and the color encoder.
Sometimes it is desirable that the amount of gamma correction be adjustable. For example, when several television cameras are used for televising a scene, such as a sporting event or during a studio production, it is important that the contrast range of the video signal supplied from each of the various cameras be matched in order to prevent noticeable variations in the midtone brightness of the reproduced image when switching among the various cameras. Preferably, the amount of gamma correction provided by each camera can be remotely controllable in order that a person at a remote location, such as in a control room, can conveniently monitor the video signal from each camera and adjust the value of their gamma correction.
Furthermore, it is also desirable to intentionally distort the contrast range of a reproduced image in, e.g. closed circuit surveillance and video microscope systems, to improve the visability of a portion of the reproduced image. An adjustable gamma controller can conveniently provide such intentional distortion of the contrast range.
In general, adjustment of the nonlinear circuits which determine the amount of gamma correction will change either one or both of the black level and the white level of the video signal. Variations of these levels in a multi-camera television production system is undesirable since it results in noticeable brightness variations in the reproduced image. In the past, relatively complex circuit arrangements were used in conjunction with adjustable gamma controllers for maintaining relatively constrast black and white video signal levels without disturbing the desired nonlinearity set by the gamma controller.
Therefore, it is desirable to provide a relatively simple and therefore more cost effective and inherently more reliable adjustable gamma controller. Furthermore, it is desirable that adjustment of the gamma controller does not adversely effect the white and black levels of the video signal.